Several sheet materials like polymeric materials, leathers, textiles, engineering composite materials etc. are being widely used in several engineering environments. As reported by Ramanathan et al (Journal of the Indian Leather Technologists Association, 16, 293, 1968), these materials undergo various stresses in mutually perpendicular directions while in use for different applications. For example, the upper leather of a shoe is subjected to force in all directions. Ramanathan et al (Proceedings of the International Southern Biomedical Engineers Conference, Shrevepor, La., USA, p. 1, 1982) reported that the force on the surface of the upper portion of leather is enormous while walking. Moreover, with the advent of fashions several combination of materials are being used in shoe manufacture. It is therefore necessary to understand the mechanical properties, stress relaxation, hysteresis and mechanism of failure of these materials by simulating the user conditions for efficient use of these materials.
Presently the viscoelastic nature of materials is analysed by unidirectional testing using Universal Testing Machines (UTM), where a dumbbell shaped sample (1) gripped by two jaws (2) is pulled apart using motorized crosshead (3) till the sample fails, while the force developed during the process is sensed by a force transducer (5) and recorded by a recording device (4). As reported by Ridge and Wright (Biorheology, 2,67,1964), Muthiah et al (Biorheology, 4,185,1967), the force generated per unit area during the deformation, which eventually leads to the fracture of the sample, is usually taken as a measure of the tensile strength of the material under consideration. In addition, the gradual decrease in the force, developed during deformation, to a given strain level, viz., stress relaxation, provides information on the rearrangements of the molecular structure within the sample, which depends on their viscous nature, as reported by Arumugam et al (Handbook of Advanced Materials Testing, Edited by N P Cheremisinoff and P N Cheremisinoff, Marcel Dekker Inc., New York, p 909, 1995). Similarly repeated deformations up to a certain strain level and back provide information about the plasticity of the sample, as reported by Vogel (Bioengineering and Skin, 4, 75,1988). Vogel (Journal of Medicine, 7, 177, 1976) has developed a theoretical model for the stress relaxation process for rat skin using uniaxial samples. As reported by Arumugam (Ph. D. Thesis, University of Madras, 1989) and Arumugam et al (Journal of Biosciences, 19, 307,1994), all these characteristics depend on the rate at which the experiments are performed. Ambrazyavi et al (Pat. No. SU 1226123 dt. 23/411986) have designed an apparatus to measure relaxation process after compression in polymers. Dzhunisbek et al (Pat. No. SU 998918 dt. 23/4/1983) have also fabricated a device to measure the compression based stress relaxation coupled with friction in polymers. Methods involving monitoring of fall in pressure to detect relaxation have also been attempted for polymers as disclosed in Pat. No. SU 354317. Modifications induced during the relaxation process such as mechanical reduction in the thickness of the sample have also been tried by Dubovik et al (Pat. No. SU 1186996 dt. 23/10/1985). The main limitation in all the above devices is that they are all designed to measure the relaxation process only along a single axis or dimension.
In actual user conditions materials are generally exposed to forces acting in more than one dimension, which includes bending and stretching. Hence, the use of unidirectional sample for predicting the conditions for failure does not simulate the actual user conditions. In other words, deduction of realistic information and predicting or computing the mechanism of the failure of these viscoelastic materials from unidirectional tests has its limitations because of their high Poisson's ratio. For example, there is tremendous amount of lateral contraction due to the high Poisson's ratio of the leather while testing the sample in the conventional UTMs. Therefore, the extension and the breaking load observed for the sample under unidirectional test conditions are very different from the shoe undergoing a similar kind of stress during usage. Moreover, intermediate conditions of testing and the formation of surface cracks and their role in hastening or delaying failure during application cannot be studied by hitherto known techniques. In addition, there are inherent inhomogeneities in certain materials like leather, textiles etc., which necessitate performing uniaxial testing in more than one direction to get a complete understanding of the material properties.
The main object of the present invention is to provide a two dimensional stress relaxation testing device, which obviates the drawbacks stated above.
Another object of the present invention is to analyze stress related behavior like relaxation, hysteresis, fatigue and creep of materials simultaneously in two mutually perpendicular directions.
Yet another object of the present invention is to perform dynamic testing of materials Still another object of the present invention is to reduce the number of test samples and time as well as labor while analyzing the stress related properties of materials.
Yet another object of the present invention is to study the mechanical properties of sheet materials without any lateral contraction.
Still another object of the present invention is to develop a device to provide accurate information on the failure of the materials as the lateral contraction is not present.